1. Field of Invention
The present invention relates to the field of safety inspection technology, in particular to a drift tube structure used in inspection equipment for inspecting drugs and explosives by means of ion mobility technique.
2. Description of Prior Art
Ion mobility spectrometer discriminates different ions according to a fact that different ions have different drift velocities in a uniform weak electric field. The ion mobility spectrometer is usually formed of a sample input section, an ionization section, an ion gate or an ion storage section, a migration zone, a collection zone, a sensing circuit, a data acquisition and processing and control section, etc.
In the existing techniques, to achieve a higher ion transmittance, attention has been paid to ion focusing technology. A time-variable electric field is used in focusing in U.S. Pat. No. 4,855,595. Cup-shaped electrodes are used in focusing to increase ion transmittance in U.S. Pat. No. 5,189,301. Unfortunately, a considerable quantity of ions may be scattered to the electrodes and disappear. In U.S. Pat. No. 6,727,495, a time-variable electric field, for which electrode sheets are divided in even number with a phase difference of 90 degrees between adjacent electric fields of divided sheets, is coupled with a linear electric field to focus the ions and thus increase ion transmittance. This technique has the disadvantage of complex structure and control. In U.S. Pat. No. 7,223,969, a method of alternating between strong and weak static electric fields is adopted in ion focusing to increase ion transmittance. Again, this method results in complex structure.
FIG. 1 shows a drift tube structure in the prior art. It comprises a sample input unit 1, a semi-permeable film, an ionization source 2, an ion gate 3, a drift tube 4, a Faraday tray and a front circuitry 5, etc., arranged in this order. The internal electric field of the drift tube is susceptible to interference, and the peripheral ions are often scattered and disappear due to the electric field having a non-uniform edge.